Hollow cathode sputtering device

ABSTRACT

AN APPARATUS FOR THE DEPOSITION OF THIN FILMS OF MATERIAL UPON ARTICLES BY CATHODIC SPUTTERING. DEPOSITION OCCURS WHEN A FLOW OF APPROPRIATE CARRIER GAS CARRIES MATERIAL SPUTTERED FROM THE INTERIOR OF THE HOLLOW CATHODE TO THE ARTICLE.

Aug. 20, 1974 GRUEN ETAL 3,830,721

HOLLOW CATHODE SPUTTERING DEVICE Filed Aug. 22, 1973 United States Patent O 3,830,721 HOLLOW CATHODE SPUTTERING DEVICE Dieter M. Gruen, Downers Grove, 11]., Dean H. W.

Carstens, Los Alamos, N.M., and John F. Kozlowski,

West Lafayette, Ind., assignors to The United States of America as represented by the United States Atomic EnergyCommission Filed Aug. 22, 1973, Ser. No. 390,644

Int. Cl. C23c 15/00 US. Cl. 204-298 4 Claims ABSTRACT on THE nrscLosUnE An apparatus for the deposition of thin films of material upon articles by cathodic sputtering. Deposition occurs when a flow of appropriate carrier gas carries material sputtered from the interior of the hollow cathode to the article.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to an apparatus for depositing thin films of material upon articles, and more particular- -ly to an apparatus for depositing thin films of material by cathodic sputtering.

In recent years there has been a considerable interest generated in thin films and in improved methods for preparing such films by cathodic sputtering techniques. Unfortunately there are some problems associated with cathodic sputtering.

For example, cathodic sputtering is generally uneconomical and wasteful of film material since only a small portion of the material sputtered is actually applied to the desired surface or substrate to form the thin film. While this waste is not particularly important when relatively inexpensive materials such as nickel or copper are involved, the cost of the waste material may be prohibitive when films of precious metals such as gold, silver or platinum are desired. Also, since most methods for the deposition of thin films require high temperatures, undesirable impurities in the thin-film coating may result.

SUMMARY OF THE INVENTION We have developed an apparatus for the deposition of thin films of material on an article, which is simple and eflective, which wastes little or none of the material to be deposited and which will produce thin films of a very high purity. The invention is an apparatus of the hollow cathode type and has a cylindrical hollow cathode electrode of the material to be sputtered, an anode electrode positoned within'the hollow cathode, means for establishing an electrical potential ditferential between the cathode and the anode for sputtering material from the cathode and means for directing a flow of gas through the center of the hollow cathode to carry the sputtered material to the article to be coated.

The apparatus of this invention has several advantages over the prior art apparatus. The flow of sputtered material from the hollow cathode to the substrate of the article to be coated is 'very directional. Therefore, all or nearly all the material sputtered from the cathode is carried to the substrate to form the thin-film coating. Since the flow of gas carrying the sputtered material is directional, the area to be coated can be readily controlled so that with proper masking only the desired portion of the substrate will be coated with a thin film. The article to be coated is not appreciably heated by 3,830,721 Patented Aug. 20, 1974 the apparatus. Therefore it may be useful for the application of thin-film coatings on heat-sensitive articles such as biological specimens where heating would dry and distort the specimen and thus render it useless for investigative purposes, such as viewing with an electron microscope. It might also be noted that the apparatus is of small size and has low power requirements and is useful for coating small articles in applications where large and bulky equipment would not be desirable.

It is therefore one object of the present invention to provide anapparatus of the hollow cathode type for coating an article with a thin film of material.

It is another object of the invention to provide an apparatus for coating an article which is not wasteful of coating material.

It is a further object of the invention to provide an apparatus for coating an article in which the area to be coated can be readily controlled.

Finally, it is the object of the invention to provide an apparatus for coating an article with a thin film of material which will not appreciably heat the article to be coated.

BRIEF EXPLANATION OF THE DRAWING The drawing is a cross-sectional view of the apparatus of the invention.

As shown in the drawing, the sputtering device of the invention, designated generally as 10, consists of a vacuum chamber 12, a short tubular casing 14 extending from one wall of the chamber having a first flange 16 at the end thereof, a body tube 18 of smaller diameter than said casing having a second flange 20 at one end for connecting with flange 16 and a socket connection 22 at the other end, and a bifurcate tube 24, of dielectric material, connected in a pressure-tight seal to socket 22 of tube 18 through a ball connection 26. Opposite ball connection 26 bifurcate tube 24 is connected at 28 with a source of an appropriate carrier gas (not shown) which is regulated by gas valve 30. A short branch tube 32 located between ball connection 26 and valve 30 is sealed by metal end cap 34.

Flange 20 is removably attached to flange 16 by appropriate fasteners 36 and is electiically insulated therefrom by a dielectric washer 38 and dielectric bushings 40 on fasteners 36. Circular grooves 42 and 44 in the mating surface of flanges 16 and 20, respectively, contain a resilient O ring 46 to provide a pressure-tight seal with washer 38.

Passing through second flange 20 in a pressure-tight seal and extending into casing 14 in a converging relationship are straight cooling tubes 48, which are fixedly attached opposite each other to one end of elongated cylindrical cathode holder 50. Holder 50 is centered on the longitudinal axis of casing 14 with the end opposite tubes 48 extending into vessel 12. The inner surface 52 of holder 50 is threaded to threadedly engage threaded outer surface 54 of elongated cylindrical hollow cathode electrode 56. Cathode electrode 56 has a smooth surface 58, a first open or discharge end 60 directedat an article to be coated 62 located within vessel 12 and a second open end 64. First end 62 also has an outward turned lip 66 to ease removal of cathode 56 from holder 50. Tubes 48 communicate with an annular passage -68 in holder 50 through which a coolant, such as water, may be circulated to control the temperature of "holder 50 and cathode 56.

Small elongated tube 70 of dielectric material passes through the center of body tube 18 and seal'72 in ball connection 26 and has a first end in bifurcate tube 24, short of branch tube 32, and a second end in the center of cathode 56 for directing flow of gas through the center of the cathode. Located within tube 70 is anode electrode wire 74 which extends from the second end of elongated tube 70 and out the first end into branch tube 32 where it is connected to end cap 34 to provide an electrical contact therewith.

The general method of operation of the invention is as follows:

The article for coating 62 is mounted within vessel 12 about 2 to 3 cm. distant and in line from the discharge end 60 of hollow cathode 56 which has been prepared of the metal to be coated, and a vacuum of about 10- to 10- Torr is established within the vessel. A flow of coolant is circulated through tubes 48 to annular passage 68 to cool the temperature of the hollow cathode 56 and cathode holder 50. A flow of an appropriate carrier gas is estabblished through bifurcate tube 24, adjusted with valve 30 and the gas passes through elongated tube 70 into the center of hollow cathode 56 to article 63. An electrical potential difference sufiicient to sputter metal atoms from cathode 56 is then established between anode wire 74 and cathode 56 using metal end cap 34 as one contact and tube 48 as the other electrical contact, whereby metal atoms are sputtered from hollow cathode 56 and are carried by the flow of gas to the article to be coated 62 which is thus provided with a thin film of the desired material.

The apparatus of this invention is useful for coating any metal which may be sputtered such as, for example, calcium, nickel, niobium, aluminum, vanadium, iron and copper, and may be useful for coating any substrate which may be coated by other methods. This is accomplished by placing a cathode electrode of the desired metal in the cathode holder. In the case of expensive metals such as platinum or gold, it has been found that a thin metal foil tube placed in a copper cathode works very well to provide sputtering material. In order to provide thin films of extremely high purity, it is preferred that the anode wire should be of the same metal as is the cathode.

The appropriate gas is generally a noble gas such as argon or helium, although gases such as nitrogen may be used when sputtering gold or other unreactive metals. A small amount of reactive gas may be added to the carrier gas to cause chemical sputtering in order that a compound of sputtering material might be used as the thin-film coating. For example, one to two percent of a reactive gas such as oxygen, nitrogen or hydrogen sulfide would be sufficient to provide a coating of an oxide, nitride or sulfide to the article to be coated. The flow rate of gas through the hollow cathode electrode will depend upon the internal diameter of the electrode. It must provide sufficient pressure within the electrode, about 0.2 to 0.3 Torr, to establish and sustain the glow discharge created by applying the electrical potential diiferential between the two electrodes.

The amount of vacuum in the chamber containing the article to be coated is not critical, but should be maintained at a minimum of about Torr in order to sustain the coating operation.

The amount of electrical potential difference which must be applied between the anode and the cathode will vary with the material being sputtered and with the rate of gas flow. In general, a range of 50 to 100 milliamperes are sufficient at an applied voltage of from 300 to 450 volts.

The following example is illustrative of the apparatus of the invention and is not to be taken as limiting the extent of the invention as defined by the claims appended hereto:

EXAMPLE A hollow cathode of niobium metal about 1.125 inches long and having an internal diameter of 0.086 inch was prepared and inserted in a hollow cathode support as hereinbefore described. A sodium chloride plate substrate was prepared and mounted in the vacuum chamber about 2 to 3 cm. distance from the discharge end of the hollow cathode. A fiow of coolant water to the ca hode holder was commenced and a flow of argon gas was started at a rate from about 0.0022 to 0.004 grams per minute and a high voltage power supply rated at 500 milliamperes was set for milliamps at 400 volts to produce a soft glow which illuminated the discharge end of the cathode. After a period of about 15 minutes, the apparatus was deactivated and the sodium chloride plate was inspected and found to have a very even, highly polished mirrorlike surface, indicating that it had been evenly coated with niobium from the niobium hollow cathode.

As can be seen from the above example and from the foregoing description, the apparatus of this invention provides an effective and efiicient means for applying thinfilm coatings upon articles which must be coated.

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. An apparatus of the hollow cathode type for coating an article by sputtering comprising:

a vacuum chamber including means for supporting the article to be coated;

an elongated cylindrical hollow cathode electrode, having an inner surface and first and second open ends, said first end being directed at the article to be coated and being within said chamber;

support means for holding said cathode in the chamber, including means for cooling the support and cathode;

an anode electrode positioned within the cathode, along the major axis thereof and extending out of the cathode through the second end;

means for providing a flow of gas through the cathode toward the article to be coated; and

- means for providing an electrical potential differential between the anode and the cathode for sputtering metal atoms from the inner surface of the cathode which are carried by the flow of gas to the article to be coated.

2. The apparatus of claim 1 wherein said cathode support means comprises an elongated cylindrical hollow cathode holder, within the chamber, surrounding the cathode, said holder having an annular passage;

a pair of tubes attached to the end of the cathode holder opposite the first end of the cathode and extending out of the vacuum chamber for supporting the holder and cathode, said tubes being in communication with the annular passage in the cathode holder for the circulation of a coolant therethrough for cooling the cathode and holder.

3. The apparatus of claim 2 including an elongated tube of dielectric material which extends from within the hollow cathode electrode out the second open end for enclosing the anode electrode and for directing the flow of gas through the cathode.

4. An apparatus of the hollow cathode type for coating an article by sputtering comprising:

a vacuum chamber including means for supporting the article to be coated; said vacuum chamber having a wall;

a short tubular casing forming an extension of said vacuum chamber extending from one wall of the vacuum chamber, having a first flange on the end thereof;

a body tube of smaller diameter than said casing having a second flange on one end and a socket connection on the other end, said second flange being removably attached to and electrically insulated from said first flange;

a bifurcate tube of dielectric material having a ball connection on one end and an opposite end, said ball connection being connected in a pressure-tight seal to the socket connection, the opposite end being adapted for connection to a source of gas, said tube also containing a valve for controlling the fiow of gas there; through, and a short branch tube, between the ball connection and the valve, ending in a metal end cap;

a pair of straight tubes extending in converging relationship through a pressure-tight seal in the second flange into the tubular casing;

an elongated cylindrical cathode holder in the tubular casing centered on the longitudinal axis thereof, said holder having opposite sides of one end attached to the tubes, the holder having a threaded inner surface and an annular passage in communication with the tubes for the circulation of a coolant therethrough;

an elongated cylindrical hollow cathode electrode within the holder, said cathode having a threaded outer surface, an inner surface, a first open end directed at the article to be coated and a second open end, said threaded surface threadedly engaging the inner surface of said cathode holder;

a small elongated tube of dielectric material within the body tube, said elongated tube having a first end in said bifurcate tube near the branch tube and a second end in the center of the cathode electrode and being sealed within the ball connection for directing a flow of gas from the bifurcate tube through the cathode electrode;

an anode electrode wire in the elongated tube extending from the second end of said tube out the first end into the branch tube, said wire being electrically connected to the metal end cap; and

means for applying an electrical potential differential between the anode and the cathode for sputtering metal atoms from the cathode.

References Cited UNITED STATES PATENTS OTHER REFERENCES Darbyshire, Design of an Apparatus for Cathode Sputtering, Journal of Scientific Instruments, vol. 10, 1933, pp. 8385.

20 JOHN H. MACK, Primary Examiner W. A. LANGEL, Assistant Examiner US. Cl. X.R. 

